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Piston rings are manufactured and classified based on their function and usability. Their primary purpose is to seal the chamber in which the piston moves, such as the combustion chamber of a 2-stroke or 4-stroke engine. In marine engines, three or more types of rings are typically fitted around the piston's circumference.
As a critical component of the piston, the number and type of piston rings vary depending on the engine's type and capacity.
In large 2-stroke engines, compression rings are used to seal the combustion chamber, while wiper rings are positioned below them to clear deposits from the liner and distribute oil across its surface.
In smaller marine engines, various types of piston rings serve specific purposes. For example, an oil scraper ring is used in 4-stroke engines, as these are trunk-type engines where crankcase oil directly accesses the cylinder liner and piston. This article will explore the different types of piston rings commonly used in marine engines.
Compression rings, positioned in the first grooves of the piston, provide a seal above the piston to prevent gas leakage from the combustion side. Their primary functions are to contain combustion gases and transfer heat from the piston to the cylinder walls. They also control oil by shearing the oil layer left by the oil ring, ensuring proper lubrication and supporting the sealing and heat transfer functions of the top compression ring.
Also known as the Napier ring or backup compression ring, the wiper ring is located below the compression ring. Its main roles are to clean excess oil from the liner surface and to prevent further gas leakage that escapes past the top compression ring. Most wiper rings have a tapered face angled toward the crankshaft to enable a wiping action as the piston moves. If installed incorrectly, with the tapered angle facing the compression ring, it can lead to excessive oil consumption by pushing oil toward the combustion chamber.
Oil control rings regulate the amount of lubricating oil along the cylinder walls and distribute it evenly around the liner's circumference. Also called scraper rings, they remove excess oil from the cylinder walls and return it to the crankcase. Oil rings typically include holes or slots cut into the radial center of the ring, allowing excess oil to flow back to the reservoir.
Oil rings may be one-piece or two-piece. For enhanced contact pressure between the ring and liner, chamfered edges may be added to the lands or directed toward the combustion chamber. Two-piece oil rings consist of a cast iron or profiled steel ring and a coil spring made of heat-resistant steel, which wraps around the ring's circumference to maintain consistent pressure and contact with the liner surface.
Cast iron is one of the most commonly used materials for piston rings due to its graphite content in lamellar form, which acts as a lubricant, aiding the sliding motion between the rings and the liner.
Piston rings are often alloyed or coated based on their specific function, as each ring type serves a different purpose. Common alloying elements for cast iron include chromium, molybdenum, vanadium, titanium, nickel, and copper.
To ensure durability and maximize lifespan, piston ring material is made harder than the cylinder liner.
Pistons are equipped with different types of rings in distinct grooves, each serving a specific function.
The top groove holds the compression ring, which seals the combustion chamber to prevent any gas leakage during the combustion process. When the air-fuel mixture ignites, combustion gases pressurize the piston, driving it toward the crankshaft. The gases then travel through the small gap between the cylinder wall and the piston, pressing the compression ring tightly against the cylinder liner, creating an effective seal. This sealing force is proportional to the pressure of the combustion gases.
Below the compression ring and above the oil ring lies the wiper ring, also known as the backup compression ring. Its tapered face helps to further seal the combustion chamber while wiping the cylinder liner clean of excess oil and impurities. If any combustion gases manage to pass by the compression ring, the wiper ring serves as an additional barrier.
The bottom grooves contain oil rings, positioned closest to the crankcase. These rings scrape excess oil from the cylinder liner walls as the piston moves, directing most of the removed oil back to the crankcase and into the oil sump. In 4-stroke engines, oil rings are equipped with a spring at the back to provide extra pressure, ensuring effective wiping of the liner.
The piston rings endure immense pressure from the combustion chamber. When combustion gas pressure exceeds normal levels, often due to detonation or pinging caused by a leaky injector or poor-quality fuel mixed with contaminated air, it can negatively impact ring performance.
Using contaminated fuel oil or the incorrect grade of cylinder oil also affects ring efficiency. As the rings wear down, their ability to seal combustion gases diminishes, leading to performance issues.
Common causes of piston ring wear include poor fuel or oil quality, inefficient combustion, incorrect fuel timing, or a worn cylinder liner. A typical sign of a worn ring is “blowby,” where gases pass into the crankcase or under-piston area.
Excessive wear can also cause the rings to become sticky due to carbon or sludge buildup, or lead to cracks or breakage in the rings themselves.
Inspecting piston rings is essential for ensuring their proper function and deciding if they need cleaning or replacement due to wear or damage. In 2-stroke engines, the top ring is positioned higher than in 4-stroke engines.
During regular scavenge space inspections, a screwdriver is used to press the piston rings to check for spring action or tension, which indicates the ring's integrity. Broken rings will lack this spring action.
Rings are also inspected for freedom of movement in their grooves, as carbon buildup can cause sticking, which may lead to ring breakage and liner damage. The clearance between the ring and the groove is measured to calculate wear, and scuff marks or other damage are noted.
During major overhauls, piston rings are usually replaced. Before doing so, the following conditions warrant ring replacement:
1. The piston ring is stuck in the groove.
2. The ring's axial height is reduced, leading to excessive clearance within the groove.
3. The chrome layer is damaged or peeling.
Grooves should be thoroughly cleaned to remove carbon deposits and inspected for any groove damage.
Before installing new rings, they should be rolled within the grooves to confirm that the groove depth exceeds the radial ring width. When inserting the piston with new rings into the liner, a well-lubricated piston ring compressor tool should be used to prevent sticking.
In the case of worn liners, the butt gap should also be checked. For smaller pistons, a piston ring filer can be used to adjust the gap, but for marine engines, the rings may require reconditioning at a shore workshop if the butt gap is unusual.
Finally, rings should be fitted using a ring expander and checked for proper orientation and markings. Axial and radial clearances are measured and recorded to monitor wear over time.
Before installing a piston ring, verify the marking on the new or replacement ring and compare it with the old ring to ensure it matches the correct grade and position. If the old ring's marking is unreadable, refer to the manual to identify and place the ring in its proper groove.
Clean the piston groove thoroughly to remove any carbon or sludge. Take care to avoid damaging the groove coating, as some grooves are coated with a protective film. Avoid using chipping or grinding tools that might scratch or remove this coating.
After cleaning, use a piston ring expander tool to install the ring. This tool allows the ring to expand for easy insertion into the groove from the top of the piston head. Be sure to align the ring so that the marking (“TOP” or identification number) faces upward. If no specific marking is visible, the stamped side should face up.
Use the expander tool with caution, as improper use can damage the ring or pose an injury risk due to the ring's constant tension. For smaller 4-stroke engines, if the tool is unavailable, rags can be used to expand the ring by pulling on each side to allow installation from the piston's top.
After fitting the rings, make sure the openings (butt ends) of each ring are staggered to prevent gas leakage.
Like other engine components, piston rings have a specific overhaul and replacement interval. Their lifespan depends on the ring type, engine size, and operating conditions.
- Large Two-Stroke Engines: In engines with approximately 900 mm bore, rings typically last up to 24,000 hours. For engines around 500 mm bore, lifespan can reach around 16,000 hours.
- High-Speed Marine 4-Stroke Engines: Due to higher speeds, piston rings in these engines generally last about 8,000 hours, after which they require renewal.
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